Radio communication systems often suffer from the effects of multipath propagation, whereby a transmitted signal reaches a receiver via a plurality of distinct paths from the transmitter. One solution to this problem is antenna diversity, in which two or more receiving antennas are provided for a receiver. Provided the antennas are sufficiently separated so that the signals received at one antenna are substantially uncorrelated with those received by another, when one antenna is in a null another antenna is likely to be able to receive a good signal. An example of a radio communication system which may make use of antenna diversity is a Bluetooth network, operating according to the specification defined by the Bluetooth Special Interest Group. Such a network is intended to provide low-cost, short range radio links between mobile PCs, mobile phones and other devices, whether portable or not. Communication in a Bluetooth network takes place in the unlicensed ISM band at around 2.45 GHz. At such frequencies, antenna separations of the order of a few cm are sufficient for successful diversity operation.
In an antenna diversity receiver, for example that disclosed in U.S. Pat. No. 5,940,452, a diversity controller selects the antenna providing the best signal according to a signal quality measurement, which is most commonly the RSSI (Received Signal Strength Indication). Other measures of channel quality can be used, for example checksums are used in certain cases in a DECT (Digital Enhanced Cordless Telecommunications) system. In a radio communication system in which data is transmitted in packets, it is preferable for the diversity controller to select the optimum antenna on a packet-by-packet basis. This is particularly the case in a frequency-hopping system such as Bluetooth, because successive packets will be sent on different frequencies whose characteristics will not be correlated.
However, implementation of antenna diversity on a packet-by-packet basis requires measurement of the signal quality from each antenna in turn (unless a plurality of receivers is provided, which is not generally a practical solution). The sequential RSSI measurement process employed in known receivers may therefore take too long, particularly if the preamble to each packet is short (for example, that in Bluetooth is only 4 μs long). Accordingly, it was proposed in patent application WO 0203570 to provide an antenna diversity receiver enabling simultaneous comparison of signal quality from two antennas without the need for a plurality of receivers. This involved modifying the switch which selects either the signal from the first antenna or the second antenna, and couples that signal to both in-phase and quadrature channels. The modified switch is able to route the signal from the first antenna to one of the channels and route the signal from the second antenna to the other of the channels. The resultant signals are folded around zero frequency, and therefore cannot be demodulated, but valid signal strength measurements may still be taken for each antenna. A diversity controller is able to compare the signal qualities from the two antennas during a preamble in the transmitted data and to determine which antenna to use for the remainder of the data.
The same patent application indicates that in low-IF (Intermediate Frequency) architectures employing a polyphase filter rather than low pass filters, additional circuitry would be needed for deriving a signal from each of the I and Q channels before they pass through the polyphase filter. Each of these derived signals would then need to be passed through a separate channel filter, to filter out adjacent channel signals, before signal quality measurements could be made. This involves more cost and complexity. In any case, the selection of which antenna to use is still poor because the preamble is so short that the selection is heavily susceptible to random noise during the preamble.